In the 3GPP (3rd Generation Partnership Project), W-CDMA has been standardized as the 3rd generation cellular mobile communication system at present and the service has successively started. Furthermore, HSDPA (High Speed Downlink Packet Access) with a higher communication speed has been standardized and the service is to be started.
On the other hand, in the 3GPP, evolution of the 3rd generation radio access (Evolved Universal Terrestrial Radio Access; hereinafter referred to as the EUTRA) is being examined. As a downlink of the EUTRA, OFDM (Orthogonal Frequency Division Multiplexing) has been proposed. Also, as an uplink of the EUTRA, a single carrier communication system of DFT (Discrete Fourier Transform)-spread OFDM has been proposed.
The uplink of the EUTRA includes, as illustrated in FIG. 17, an Uplink Pilot Channel UPiCH, a Random Access Channel RACH, an Uplink Shared Channel UL-SCH and a Physical Uplink Control Channel PUCCH.
The downlink of the EUTRA includes, as illustrated in FIG. 17, a Downlink Pilot Channel DPiCH, a Downlink Synchronization Channel DSCH, a Common Control Physical Channel CCPCH, a Physical Downlink Control Channel PDCCH (L1/L2 (Layer 1/Layer 2) control channel), a Downlink-Shared Channel DL-SCH) (see, for example, Non-patent Document 1).
In the OFDM communication system, signals transmitted from each of a plurality of mobile stations to a base station are demodulated in a batch, and therefore, it is necessary to control arrival time of signals reaching from respective mobile stations to the base station to be constant. Although interference derived from delay can be prevented in the OFDM communication system by providing a guard interval (of, for example, 5 microseconds with a subcarrier of 15 kHz and an OFDM symbol of 70 microseconds), the interference cannot be avoided if the timing is shifted beyond the guard interval.
A random access channel, using a minimum unit of a 1.25 MHz band, is constituted so as to cope with a large number of accesses by, for example, preparing a plurality of access channels as illustrated in FIG. 18.
FIG. 18 is a diagram illustrating an exemplary assignment on radio resources of a random access channel RACH, an uplink shared channel UL-SCH, an uplink pilot channel UPiCH and a physical uplink control channel PUCCH. In FIG. 18, the abscissa indicates the time and the ordinate indicates the frequency. Also, FIG. 18 illustrates the structure of one radio frame, and the radio frame is divided into a plurality of radio resources. In this exemplary structure, each radio resource is constituted as a unit area of 1.25 MHz in the frequency direction and 1 ms in the time direction, and the random access channel RACH and the uplink shared channel UL-SCH of FIG. 17 are allocated to these areas as illustrated in FIG. 18. In this manner, the random access channel RACH uses the 1.25 MHz band as the minimum unit. Incidentally, in FIG. 18, the uplink pilot channel UPiCH is allocated dispersedly in the areas of the uplink shared channel UL-SCH with respect to each symbol or subcarrier.
The random access channel is used for the principal purpose of attaining synchronization between a mobile station apparatus (hereinafter referred to as the “mobile station”) and a base station apparatus (hereinafter referred to as the “base station”). Furthermore, consideration is made for transmitting information of several bits such as a request for scheduling of allocating a radio resource to reduce connect time between the mobile station and the base station (see, for example, Non-Patent Document 2).
In a random access, a preamble alone is transmitted for attaining synchronization. The preamble includes a signature corresponding a signal pattern representing information, and information of several bits can be specified by preparing several tens kinds of signatures. At present, transmission of 6-bit information is assumed and preparation of 64 kinds of signatures is assumed.
It is assumed that a random ID is allocated to 5 bits of 6-bit information and that information such as a reason for the random access and a path loss/CQI (Channel Quality Indicator) of the downlink is allocated to the remaining 1 bit (see, for example, Non-Patent Document 3).
FIG. 19 is a sequence chart used for explaining an exemplary procedure of a conventional random access. In the procedure of the conventional random access, as illustrated in FIG. 19, a mobile station first selects a signature on the basis of a random ID, a reason for the random access and path loss/CQI information of the downlink (step (hereinafter shortened as “ST”) 1901). Then, a preamble including the selected signature (that is, a random access preamble) is transmitted by using a random access channel (ST1902: Message 1).
When a base station receives the preamble from the mobile station, it calculates, on the basis of the preamble, synchronization timing shift between the mobile station and the base station and performs scheduling for transmitting an L2/L3 (Layer 2/Layer 3) message (ST1903). Thereafter, when it is found from the reason for the random access that the mobile station needs C-RNTI (Cell-Radio Network Temporary Identity), the base station allocates C-RNTI to the mobile station, and transmits a random access response including synchronization timing shift information (synchronization information), scheduling information, a signature ID number and the C-RNTI (ST1904: Message 2).
When the mobile station receives these information from the base station, it extracts a response from the base station including the transmitted signature ID number (ST1905). Then, the mobile station transmits an L2/L3 message by using a radio resource scheduled by the base station (ST1906: Message 3). When the base station receives the L2/L3 message from the mobile station, it transmits contention resolution for determining whether or not contention has occurred with another mobile station (ST1907: Message 4) (see, for example, Non-Patent Document 3).
A problem of such a random access is occurrence of contention caused when a plurality of different mobile stations select the same signature and the same random access channel. When a plurality of mobile stations select the same signature and perform transmission by using a radio resource block having the same time and frequency, namely, by using the same random access channel, contention occurs in the preamble (ST1902) of FIG. 19.
When the base station cannot detect the preamble (ST1902) due to such contention, it cannot transmit the response (ST1904) including the synchronization information and the like. In this case, since the mobile station cannot receive the response (ST1904) from the base station, it should perform a random access again by selecting a signature and a random access channel after a prescribed time.
On the other hand, if when the base station can detect the preamble (ST1902), the base station performs the scheduling of an L2/L3 message and calculates the synchronization timing shift for transmitting the response (ST1904) to the mobile station. However, the plural mobile stations receive the response (ST1904) from the base station. Therefore, the plural mobile stations transmit the L2/L3 message (ST1906) by using the scheduled radio resource, resulting in causing contention in the L2/L3 message (ST1906).
When the base station cannot detect the L2/L3 message (ST1906) due to such contention, it cannot transmit the response (ST1907). In this case, since the mobile station cannot receive the response (ST1907) from the base station, it should perform a random access again by selecting a signature and a random access channel after a prescribed time. In this manner, in the case where a plurality of mobile stations select the same signature and random access channel, contention may occur, and when the contention occurs, it takes time elapsing up to ST1907 of FIG. 19 at most to detect the contention.
On the other hand, in transmission of a downlink-shared channel DL-SCH, an HARQ (Hybrid Automatic Repeat Request) is employed. In the HARQ, after decoding the DL-SCH in a mobile station, ACK (Acknowledgement) is fed back to the base station when CRC (Cyclic Redundancy Check) succeeds and NACK (Negative Acknowledgement) is fed back to the base station when the CRC fails, and thus, the base station determines whether or not retransmission is to be performed. This ACK/NACK is transmitted by using a physical uplink control channel PUCCH immediately after receiving the DL-SCH. The mobile station receives the downlink-shared channel DL-SCH after receiving the physical downlink control channel PDCCH, and transmits the ACK when the CRC succeeds.
Incidentally, in the case where the mobile station and the base station are out of uplink synchronization with each other (for example, in the case of a DRX state where data transmission has not been performed for a long period of time and the mobile station has been monitoring a signal for allocating a downlink resource on a long cycle), when downlink data transmission from the base station is resumed, the mobile station cannot transmit the HARQ ACK/NACK (Hybrid Automatic Repeat Request Acknowledgement/Negative Acknowledgement) by using a PUCCH. This is because since it is out of uplink synchronization, if the HARQ ACK/NACK is transmitted, it causes interference with another mobile station. Accordingly, in resuming the downlink data transmission, it is necessary to attain uplink synchronization through a random access.
In this case, contention cannot be avoided because a random access is performed, and it is apprehended that it may take a long time to resume the downlink data transmission. In order to avoid such a problem, a proposal for preventing contention in a random access performed for resuming downlink data transmission by using a signature for resuming the downlink data transmission has been made (see, for example, Non-Patent Document 4). At this point, the procedure for resuming downlink data transmission proposed in Non-Patent Document 4 will be described with reference to FIG. 20.
When it is determined to resume downlink data transmission to a mobile station out of uplink synchronization, a base station transmits an uplink synchronization request to the mobile station as illustrated in FIG. 20 (ST2001). The uplink synchronization request is transmitted by using an L1/L2 (Layer 1/Layer 2) physical downlink control channel PDCCH. The uplink synchronization request includes a signature ID number of a random access to be sent from the mobile station. In the following description, this is designated as a dedicated signature.
When the uplink synchronization request is received from the base station, the mobile station transmits a preamble (a random access preamble) including the dedicated signature received in the uplink synchronization request by using a random access channel to the base station (ST2002). When the preamble including the dedicated signature is received from the mobile station, the base station transmits a TA (Timing Advance) command corresponding to a synchronization timing shift to the mobile station as a response (a preamble response) to the random access (ST2003).
After transmitting the TA command, the base station transmits an L1/L2 control channel including downlink resource allocation to the mobile station (ST2004). Subsequently, the base station transmits downlink data to the mobile station (ST2005).    Non-Patent Document 1: R1-050850 “Physical Channel and Multiplexing in Evolved UTRA Uplink”, 3GPP TSG RAN WG1 Meeting #42 London, UK, Aug. 29-Sep. 2, 2005    Non-Patent Document 2: 3GPP TR (Technical Report) 25.814, V7.0.0 (2006-06), Physical layer aspects for evolved Universal Terrestrial Radio Access (UTRA)    Non-Patent Document 3: 3GPP TS (Technical Specification) 36.300, VO. 90 (2007-03), Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN), Overall description Stage 2    Non-Patent Document 4: R2-062165 “UL Synchronization”, 3GPP TSG RAN WG2 Meeting #54 Tallinn, 28 Aug.-1 Sep. 2006